Recovery of aromatics by extraction or extractive distillation with solvent mixturesof n-methyl pyrrolidone and diglycol amine



Dea 10. 1968 KARL-HEINZ EISENLoHR ETAL 3,415,739

RECOVERY oF AROMATIGS BY EXTRAGTION 0R EXTRACTIVE DISTILLATION WITH SOLVENT MIXTURES 0F N-METHYL PYRROLIDONE AND DIGLYCOL AMINE Filed Dec. 20, 1967 3 Sheets-Sheet l- J0g /0- o a 3 4.56799 l s 4.56789 /o /o' D60 10, 1968 KARL-Hamz EISENLOHR ETAL 3,415,739

\ RECOVERY 0F AROMATICS BY EXTRACTION OR EXTRACTIVE DISTILLATION WITH SQLVENT MIXTURES OF NMETHYL PYRROLIDONE AND DIGLYCOL AMNE Filed Dec. 20, 1967 3 Sheets-Sheet 2 .SELECT/l//Ty--r il U, m \1 0005 De@ l0, 1968 TIrARm-lel EISENLOHR ETA 3,415,739

RECO Y OF AROMA Y X R OR X RACTIV ILLATION WITH VENT MIXTURES N THYL ROL NE AND GLY OY AMIN Filed Dec. 20, 1967 5 eetset Z United States Patent O ABSTRACT OF THE DISCLOSURE Aromatic hydrocarbons are extracted with solvent from hydrocarbon mixtures containing both armatic and nonaromatic hydrocarbon components using a mixture of N- methyl pyrrolidone and diglycol amine as the solvent. The extractions are conducted by liquid-liquid extraction or by extractive distillation.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation in part of copending application Ser. No. 490,818, entitled Recovery of Aromatics by Extraction or Extractive Distillation with Solvent Mixtures, and filed September 28, 1965, now Patent No. 3,366,568 in the names of Karl-Heinz Eisenlohr and Eckart Mller.

BACKGROUND OF THE INVENTION Field of the invention The present invention provides an improved process for the recovery of aromatic hydrocarbons, especially benzene, toluene and xylene, from hydrocarbon mixtures containing the same, by extraction with water free solvent mixtures selective for aromatics which as a whole boil over the boiling point range of the aromatics to be recovered. The extraction with the solvent mixtures concerned is by liquid-liquid extraction or extractive distillation.

Description of the prior art The recovery of aromatic hydrocarbons from hydrocarbon mixtures by liquid-liquid extraction or by extractive distillation with selective solvents has been practiced on a cominercial scale for several decades. However, from the fact that again and again new solvents are suggested yand patented for this purpose, it can be concluded that `the ideal solvent has as yet not been found.

In addition to pure solvents, in many instances organic solvent mixtures have been proposed as the selective solvents. Furthermore, widely spread practice is to add certain quantities of water to the solvent. The addition of water offers the following advantages:

(1) The selectivity of the solvent is increased.

(2) The separation, especially of the higher aromatics, from the extract obtained is easily accomplished even with a small ydierence between the boiling point of the solvent and the aromatics because the aromatics ydistill azeotropically with the water at temperatures under 100 C.

(3) The sump temperature in the distillation column employed for separation of the aromatics from the solvent extract is lowered so that not only are losses engendered by thermal decompositions reduced but also the heat energy required for heating the solvent to the sump temperature is lowered.

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3,415,739 Patented Dec. 10, 1968 ice (4) With regard to liquid-liquid extractions a further advantage is that the field of the existence of two phases is increased, so that larger quantities of aromatics may be contained in the feed stock without leaving the twophase field.

On the other hand, the following disadvantages of the addition of water must be taken into consideration:

(l) The solvent capacity for aromatics is reduced.

(2) The water is vaporized azeotropically with the raromatics to be separated from the solvent which causes substantial increase in the heat energy required.

(3) In many instances the water causes undesired chemical reactions, especially hydrolysis.

A number of non-aqueous solvent mixtures have also become known, such as, for example, mixtures of SO2, ethylene glycol and formamide or mixtures which consist of a primary solvent which contains glycol yderivatives and 1a secondary solvent such as methanol, ethanol `and acetone.

These solvent mixtures, however, have the disadvantage that a portion thereof has a lower boiling point than the aromatics to be recovered whereas the other portion ha-s a higher boiling point. The separation of the aromatics from the solvent therefor requires substantial plant investment as well as high operation costs which in general are not compensated for by the advantages of the solvent combinations.

Further proposals for solvents include, for example, (l) the combination of various glycol derivatives, (2) mixtures of two solvents of which the rst contains 1 or 2 hydroxyl groups and the second contains 2 or more hydroxyl groups, (3) ethylene carbonate with additions of glycerol, ethylene glycol, pentaerythritol, formamide, formic acid, ethanol amines, monochlorohydrin, acetarnide, resorcinol or hydroquinone as diluent, (4) alkane dinitriles, dimethyl hydration, N-alkylpyrrolidones, butyrolactone, cyan ethers of diethylene glycol, trior tetraethylene glycols or any desired mixtures of these solvents which are selective for aromatics or olens.

However, without exception, no advantages over the use of a single solvent have been proved for these :and many other proposals for the use of mixtures of various selective solvents boiling 'above the aromatics to be recovered for the liquid-liquid extraction or extractive distillation of aromatics, nor have any indications been given as to what quantity and in what proportions the various components of the mixture should be used in order to attain advantages over the use of single solvents.

The state of the art concerning the possibilities of use of solvent combinati-ons for the liquid-liquid extraction or extractive 'distillation of aromatics can be summarized in that the effect of the addition of water to a solvent is known qualitatively, and that something is known concerning the properties of various `combinations of glycol derivatives. However, from which points of view, solvents may otherwise be combined in order that they give good properties for liquid-liquid extraction or extractive distillation of aromatics has neither been described in scientific literature nor in the patent literature.

SUMMARY OF THE INVENTION Vof N-methyl pyrrolidone and diglycol amine.

3 BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: l

FIG. 1 is a diagram showing the maximum quantity of benzene in the heavy phase (solvent phase) with respect to the composition of the solvent mixture N- methyl pyrrolidone (hereinafter referred to as NMP) land diglycolamine;

FIG. 2 is a diagram showing the dependency of the selectivity of the composition `of solvent mixture NMP and diglycol amine;

FIG. 3 is a diagram in which the selectivity is plotted against the partition-coefficient in dependence on the composition of sol-vent mixture NMP and diglycol amine;

FIG. 4 is a phase diagram for the 3 component system NMP-diglycolamine-benzene;

FIG. 5 is a diagram showing the dependency of the viscosity at 50 C. of solvent mixture NMP and diglycol amine on its composition;

FIG. 6 schematically shows an apparatus for carrying out a liquid-liquid extraction according to the invention; and

FIG. 7 schematically shows Ian apparatus for carrying out an extractive distillation according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the invention, it was unexpectedly found that despite the large number of solvents known for these purposes and despite their very different types of chemical structure, certain rules exist, the application of which renders it possible to select from the boundless large number of possible combinations, those which can be expected to have optimal properties.

Above all the following physical properties are desired in a selective solvent for the recovery of aromatics by liquid-liquid extraction or extractive distillation:

( 1) High selectivity.

(2) High capacity.

(3) High aromatics range, that is, the formation of a `two-phase system even at high aromatic concentrations.

(In the following, the highest aromatics concentration in the 3 substance system benzene-n-heptane-solvent at which a two-phase system is still present and above which separation into two phases no longer occurs is indicated as the aromatics range.)

(4) Low solubility of the solvent in the hydrocarbon phase.

(5) Low viscosity.

(6) High density.

(7) Boiling point above the boiling point range of the aromatics to be recovered.

(8) 'Ihermal and chemical stability at boiling point.

(9) Melting point below ambient temperature.

It was ascertained that these various physical properties do not `occur with complete irregularity in the individual solvents but rather that certain conformities to rules be observed which are of special significance for the selection of a favorable solvent combination.

Of the properties given above, the third, namely, the aromatics range is especially suited as the classifying principle for dividing the solvents into certain groups. If the solvents are arranged according to increasing aromatics range they can be divided into three groups:

(A) Solvents with low aromatics range.

(B) Solvents with high aromatics range and (C) Solvents which do not have unlimited miscibility with aromatics.

A low aromatics range is one of less than 50% by weight and preferably less than 35% by weight is the maximum of the two-phase region.

The solvents of the first group A are characterized by their high capacity, low selectivity and high solubility of the solvent in the hydrocarbon phase. In view of their low selectivity the solvents 0f this group normally are only employed in combination with water whereby the above-mentioned disadvantages must be accepted.

It has now been found that most of the solvents of this group possess two other important and advantageous properties, namely, low viscosity and good thermal stability at the boiling point. Compounds with heterocyclic ring systems, such as, morpholine, N-methyl pyrrolidone, furfural, keto dioxane, and also dimethyl formamide, aniline, ethylene diamine, nitromethane, ethylene glycol mono-methyl ether, diethylene glycol mono-methyl ether, and others, all of which have an aromatics range below 35%, and diethylene triamine, triethylene tetramine, tetraethylene pentamine, butyrolactone, and dimethyl sulfoxide with aromatics ranges between 35 and 50% especially are illustrative of the group of solvents with low aromatics r-ange.

The solvents of group B with a high aromatic range, that is, solvents with an aromatic range of at least 50% in the maximum of the two-phase system belong to the second group. They are therefore near the solubility limit with aromatics, for example, so that they are miscible in all proportions with benzene but not with xylene.

There are solvents of this group which have very high selectivity with good capacity, the latter, nevertheless, being lower than that of the solvents of group A. As a consequence, solvents of group B would be the ideal solvents for the extraction of aromatics insofar as `only the selectivity and capacity is taken into consideration, but remarkably they without exception possess other physical properties of such unfavorable values that they must be considered substantial disadvantages. Above all, representatives of this group all have the disadvantages that they are no longer stable at their boiling point and have a high viscosity. In addition, their melting points in some instances are above normal temperature.

Compounds with double bonded oxygen and dinitriles, such as, for example, sulfolane, ethylene and propylene carbonate, oxy, thioand imino-dipropionitrile, monomethyl formamide and tetraethylene glycol especially are illustrative of group B solvents.

The solvents of the third group C have a miscibility gap with aromatics, that means, they are not miscible in all proportions with aromatic hydrocarbons. Representatives of this group which dissolve less than 25% of benzene at 20 C. are preferably employed. Their capacity is less than that of the solvents of both other groups. Their selectivity is better than that of solvents of the first group. Almost all have a high viscosity. Thermal stability at the boiling point is sometimes given. This group mostly contains chain compounds with one or Kmore hydroxyl groups, often also compounds with amino groups, such as, for example, ethylene glycol, diethylene glycol, propylene glycol, glycerol, mono, diand triethanol amine, 1.4- cyclohexane dimethanol, diglycol amine, formamide, malondinitrile, hydrazine and others.

According to the invention it was found that solvent mixtures could be obtained which have substantially better properties than could be expected from the rule of mixtures by mixing a solvent having a low aromatic range belonging to the first group (A) with a solvent having a miscibility gap with aromatics and belonging to the third group (C). In each instance the selectivity is better than could be expected from the rule of mixtures. The ability to take up aromatics with solvents of group A is limited by their aromatics range, and with solvents of group C by their low solvent capacity for aromatics. The combination of a solvent of group A with a solvent of group C results in a mixture having an ability to take up aromatics which is always better than that of each component and which often exceeds that of the component with the best take up ability by 50% and more.

An undesirable characteristic of solvents of group C is their high viscosity. In this respect, an unexpected advantage also is found in the combinations according to the invention as the viscosity of the solvent mixture is always less than could be expected from the rule of mixtures.

In addition, it was found that the best mixing solution for the various solvent mixtures can quickly be found according to a very definite method:

One investigates the three component system, solvent A-solvent C-benzene, determines the solubility limits and then the critical point (plait point), that is, the point where at the solubility limit the volumes of the light and heavier phases are equal. The composition at this point is the most favorable composition for the liquid-liquid extraction or extractive distillation of aromatics.

The critical point for a particular composition of a solvent A and solvent C, with the prerequisite that solvent A and benzene and solvent A and solvent C are completely miscible and that solvent C and benzene have a miscibility gap, is determined as follows:

Equal parts by volume of benzene and solvent C are placed in a vessel. Two phases are formed, one of which predominantly contains benzene-generally the lighter phase-in the following designated as the HC-phase (hydrocarbon phase) and one which predominantly contains solvent C-generally the heavy phase-designated in the following as the S-phase (solvent phase). Then small increments of solvent A are added while observing how both phases behave or change with respect to each other. When a certain quantity of solvent A has been added the phase separation line will suddenly disappear and only one phase will be present. If by chance the volumes of the phases have been equal just before disappearance of the phase separation line the critical point would already have been determined. The critical point lies between the composition where two phases are last observed and the composition at which the phase separation line disappears. Usually, however, the volumes of the phases will be different and shortly before disappearance of the phase separation line the phase present in the larger quantity will be a multiple of the other phase. The procedure which follows under such circumstances depends upon which of the two phases was the larger just before disappearance of the phase separation line. If the HC-phase is the smaller one, small increments of benzene are added to the mixture until the mixture becomes cloudy and again separates into two phases. If at this point the HC-phase is still the smaller one, then a mixture of equal parts of benzene and solvent A are added in small incre-ments until the phase separation line again disappears so that only one phase is present. Thereafter this procedure is repeated, that is, so much benzene is added until clouding and phase separation occurs and then so much of an equal mixture of benzene and solvent A is added until the phase separation line disappears, at some point the condition will occur in which the S-phase rather than the HC-phase is the smaller. At this point the critical point has already been exceeded and one can usually determine its position by interpolating between the last and second last measuring point. However, if this is not believed sufficiently accurate, because the quantities added were selected too large, several mixtures are prepared which are near the critical point and which still just separate into two phases and a mixture of equal quantities of benzene and solvent A is added thereto in small increments. The mixture in which the two phases are of equal volume just before the phase separation line disappears corresponds to the composition of the critical point.

If, on the other hand, the S-phase is smaller just before disappearance of the phase separation line during the first addition of solvent A the procedure is analogous but the benzene added to cause phase separation is replaced by solvent C and the mixture of equal parts of benzene and solvent A added to cause disappearance of the phase separation line is replaced by a mixture of equal parts of solvent C and solvent A and the procedure repeated until the S-phase just becomes the larger phase and the critical point interpolated or determined analogously to the above procedures.

When the feed stock from which the aromatics are to be recovered is one in which the non-aromatics predominantly are parains or if `a paraffnic antisolvent is used the quantity of solvent C in the mixture: can be 5 to 10% less than previously defined. If, on the other hand, the nonaromatics in the feed stock contains large quantities of olefins and/ or naphthenes it often is expedient to increase the quantity of solvent C in the mixture about 5 to 10%.

Preferred mixtures of solvents of the Group A with solvents of the Group C are as follows:

Solv ent Components Weight mixture percent I N-methylpyrrolidone- 45. 0 Ethylene glycol. 55. 0 II N-methylpylrolidon 58. 99 Glycerol 41. 01 III N-methylpyrrolidone- 38 2 Monoet hanolamine 66. 8 IV N-methylpyrrolidone- 25. 1 Diglyeolamine* 74. 9 V Butyrolacetone..- 79. 8 Glycerol 20. 2

*M2-amino ethoxyl ethanol or oxethoxyethylamine.

These relationships are more fully explained in the following with reference to FIGS. l-7 with the mixture of N-methyl pyrrolidone (abbreciated as NMP) and diglycolamine as example. As can be seen from FIG. 1 the ability to take up aromatics of a mixture of a solvent of group A (NMP) and a solvent of group C (diglycolamine) is better than that which corresponds to the rule of mixtures and is represented by the broken line A-C. In general, the ability of a solvent mixture of group A with a solvent of group C to take up aromatics is greater than that of each solvent individually. In the illustration given the maximum take up is achieved with a mixture of 75% of diglycolamine and 25% of NMP and amounts to 70 weight percent of benzene in the heavy phase. The dependency of the selectivity upon the solvent mixture composition, once for 10% benzene and once for 40% benzene in the light phase, is shown in FIG. 2. Both curves run above the tie line between the points of the pure solvent compound.

The manner in which the optimum between selectivity and capacity can be determined is shown in FIG. 3. In such ligure the capacities (partition coefficients) and the selectivities for different mixing ratios of NMP and diglycolamine are respectively plotted as abscissa and ordinates. All data are based on a 3 component mixture of solvent-benzene-heptane at a ratio of :20:10 parts by weight in order to have comparable'results. A logarithmic scale was chosen since the yrelative change of the two characteristics is more important than their absolute change. It can be seen that the points of the individual mixtures lie above and to the right from the line A-C and therefore towards values of higher capacity and higher selectivity. It furthermore can be seen that the range which extends furthest up and right and therefore the optimal economic range lies between 70 and 80% diglycolamine.

This value of about diglycolamine is also found when the three component diagram of both solvents with benzene is investigated and the critical point is determined as described above. FIG. 4 gives the diagram for the system NMP-diglycolaminebenzene. At the critical point P the ratio of diglycolamine to NMP is 75:25.

It can also be seen from FIG. 5 that the viscosity at 50 C. of a mixture of NMP and diglycolamine, above all in the middle range, is significantly lower than was to be expected from the rule of mixtures.

The present invention is generally suited for the recovery of aromatcs from hydrocarbon mixtures containing aromatics by liquid-liquid extraction or extractive distillation and also for the recovery of extracts in which the concentration of aromatics in the hydrocarbon extracts has only been increased over that in the starting mixture. An especial advantage of the process according to the invention is that it also is adapted for the recovery of high purity aromatics, which in recent times have assumed considerable signicance as starting materials for chemical syntheses.

The process according to the invention renders it possible to recover aromatics whose non-aromatics content is below 0.1% and even under 0.01% without dillculty. Even higher purity requirements such as absence of nonaromatics in quantities capable of chromatographic detection can be achieved according to the invention.

The invention is further illustrated by the following examples and FIG. 6 and FIG. 7 which diagrammatically show, respectively, the apparatus employed in such examples.

Example 1 2500 kg./h. of a feed stock of the following composition Wt. percent B (benzene) 4 T (toluene) 12 X (xylene) 18 C9 aromatics 9 Nonaromatic hydrocarbons 57 were supplied through conduit 1 into the 12th stage of a 24 stage countercurrent extractor 2 (a mixer settler battery) in which it was extracted with 8000 kg./h. of a solvent mixture consisting of 75 weight percent of diglycolamine and 25 weight percent of NMP. 10,000

were withdrawn from the bottom of extractor 2 and supplied to the preliminary distillation column 4 over conduit 3. Column 4 had 40 actual trays and was operated with av reflux ratio of 0.5:l. 1000 kg./h. of the head product which consisted of Wt. percent B 10 T 5 Nonaromatic hydroca-rbons 85 was recycled to extractor 2 as reflux over line 5. This head product contained all of the non-aromatics originally contained in the extract. 1500 kg./h. of a rainate of the composition Wt. percent X 1.7 C9 aromatics 3.3 Nonaromatic hydrocarbons 95 were withdrawn from the extractor through line 6.

9000 kg./h. of a non-aromatics free extract were withdrawn from the sump of column 4 through line 7 and supplied to solvent stripper 8 which contained 20 actual trays and was operated with a reflux ratio of 1:1. 1000 kg./h. of pure aromatics of the composition Wt. percent B 10 T 30 X 42.5

C9 aromatics 17.5

were taken off overhead through conduit 9. The sump product (8000 kg./h.), which was practically pure solvent mixture, was recycled to extractor 2 through line 10.

The aromatics mixture taken olf Overhead from stripper 8 was distilled in a known manner to separate it into its components. The benzene recovered had a melting point of 5.50 C., the toluene recovered had a refractive index 11D2=L4967 and no non-aromatics were discernible gas chromatographically in the xylene fraction.

Example 2 1000 kg./h. of the benzene cut of a catalytic reformate of the following composition:

Wt. percent Benzene (B) 75 Non-aromatics boiling under 75 C. (NA 75) 18 Nonaromatics boiling between 75-l05 C.

were supplied over conduit 11 to distillation column 12 with 60 trays, the introduction being on the 30th tray. 2500 kg./h. of a mixture of 33% by weight of NMP and 67% by weight of diglycolamine were supplied to the 55th tray over conduit 13. 275 kg./h. of the distillate were removed from the head of the column through conduit 14. The distillate contained practically all of the non-aromatics contained in the feed stock and was of the following composition Wt. percent B 7.3 Nonaromatic hydrocarbons 92.7

So much heat was supplied to the sump of the column that, with the withdrawal of 275 kg./h. of distillate from the head of the column, a reflux ratio of 7:1 was maintained at the head of the column.

A mixture of 2500 kg./h. of solvent mixture and 730 kg./ h. benzene was withdrawn from the sump of column 12 and supplied through conduit 15 to the 15th tray of the 30 tray column 16. 730 kg./h. of substantially pure benzene were withdrawn from the head of column 6 through conduit 17. Such benzene had a melting point of 5 .50 C.

So much heat was supplied to the sump of column 16 in the form of steam that, with the withdrawal of the 730 kg./h. of benzene from the head thereof, a reflux ratio of 1:1 was maintained at the head of the column.

The benzene free NMP-diglycolamine mixture (2500 kg./h.) which accumulated in the sump of column 16 were recycled to column 2 over conduit 13.

We claim:

1. A process for separating a hydrocarbon mixture containing aromatic and non-aromatic hydrocarbons into fractions of different degrees of aromaticity by treatment with a solvent mixture and recovering a more concentrated aromatics fraction from said mixture which comprises contacting the mixture with a water free solvent mixture which as a whole boils above the aromatics to be recovered, said solvent mixture consisting of solvent (A), N-methyl-pyrrolidone, and solvent (C), diglycol amine, the ratio of the quantity of said solvent A to the quantity of said solvent C in said solvent mixture being from X :Y to X :YilO weight percent, the ratio X :Y being the ratio of solvents A and C at the critical point in the three component system solvent A, solvent C and benzene, said solvent mixture being a mixture of 65 to of diglycol amine and 35 to 15 weight percent of N-methyl-pyrrolidone, effecting phase separation of the phases thus formed, an extract phase in which the hydro carbons extracted from said mixture has a higher aromatic concentration than said mixture and a raffinate phase less aromatic than said mixture and distilling an aromatics rich 9 hydrocarbon fraction from the solvent mixture in said extract phase.

2. The process of claim 1 in which said hydrocarbon mixture and said solvent mixture are contacted with each other in a iiquid1iquid extraction process.

3. The process of claim 1 in which said hydrocarbon mixture and said solvent mixture are contacted with each other in an extractiva distillation process.

No references cited.

DELBERT E. GANTZ, Primary Examiner. H. LEVINE, Assistant Examiner. 5

U.S. C1. X.R. 208--3l3; 260-674 

